Recentemente, nanopartículas (NP) de FePt de fase tetragonal de face centrada (fct) em sistemas auto-organizados tem sido sintetizados como potenciais candidatos para gravação magnética de ultra-alta densidade (UHMR), devido a sua elevada anisotropia magnetocristalina. Entretanto, o alinhamento magnético de NP esféricas tem sido constantemente um problema, pois os eixos de fácil magnetização das NP permanecem randomicamente orientados nas três direções. Na tentativa de superar os problemas decorrentes da falta de alinhamento magnético, trabalhos recentes sugerem que NP alongadas em sistemas auto-organizados orientados perpendicularmente a superfície do substrato como possível solução para a textura e alinhamento magnético. Até o presente momento, as sínteses de nanobastões (NB) e de nanofios (NF) de FePt reportadas na literatura levam a obtenção de NP na fase cúbica de face centrada (fcc), a qual possui baixa anisotropia magnetocristalina e se mostrou termicamente instável, resultando em NP esferoidais após o processo de recozimento para a transformação da fase fcc para fct. Neste trabalho, NP de Fe55Pt55 com tamanho e composição química controlada e morfologia de nanofios e nanobastões foram sintetizadas através da adequação do método do poliol modificado. Além disso, as condições de sínteses foram ajustadas para a obtenção direta das NP na fase fct. Sistemas auto-organizados de NB orientados longitudinalmente ou perpendicularmente em arranjos hexagonais compactos foram obtidos utilizando, respectivamente, ácido oléico (AO) e oleilamina (Oam) ou hexadecilamina (HDA) como agentes espaçadores. Sistemas de NB orientados perpendicularmente com alinhamento magnético devido à anisotropia de forma combinado com as propriedades magnéticas resultantes da fase parcialmente tetragonal obtida sugerem um forte candidato a aplicação em UHMR com densidade de gravação magnética na ordem de terabits/pol2. / Recently, self-assembled FePt nanoparticles (NP) in face-centered tetragonal phase (fct) has been synthesizes as potential candidates for Ultra-High Magnetic Recording Systems (UHMR) due to high magnetocrystalline anisotropy. However, the magnetic alignment of these self-assembled fct-FePt nanoparticles also has been a constant limiting factor, since the axes of easy magnetization of spherical nanoparticles are presented randomly oriented in three directions. Concerning to avoid these restrictions arising from magnetic alignment, one-dimensional NP perpendicularly oriented on the magnetic medium surface has been reported as a possible solution for texture and magnetic alignment. To date, the reported nanorods (NR) and nanowires (NW) synthesized in the literature showed face centered cubic phase (fcc) which present poor magnetocrystalline anisotropy, instead the fct desirable phase, and were found to be thermally unstable resulting in spheroidal NP after annealing required to phase transformation from fcc to fct. In this work, Fe55Pt45 NP with morphologies varying from nanospheres, NR with different lengths up to NW were synthesized by a modified polyol process. Furthermore, the synthesis conditions were adjusted to obtain FePt NP directly in fct phase. Either longitudinal or perpendicular self-assembled NR systems in a hexagonal compact array were obtained using oleic acid (OA) and oleylamine (Oam) or hexadecylamine (HAD), respectively, as molecular organic spacers. Perpendicularly oriented NR system led to magnetic alignment due to shape anisotropy and combined with the magnetic properties due to partial tetragonal phase suggest a strong future candidate to ultra-high magnetic density recording applications capable to achieve density of Terabits/in2.

Les nanotubes de carbone (NTCs), en raison de leurs propriétés exceptionnelles et d’une utilisation potentielle dans un grand nombre d'applications, constituent surement la classe la plus étudiée des nanomatériaux. Les NTCs fonctionnalisés, qui peuvent être facilement manipulés et modifiés par liaison covalente ou fonctionnalisation non covalente, apparaissent comme de nouveaux outils dans le domaine des biotechnologies et en biomédecine. En effet, les NTC ont des propriétés optiques, électroniques et mécaniques qui peuvent être exploitées dans des applications biologiques ou biomédicales. Les nanoparticules magnétiques métalliques (NPMMs) de la série 3d ainsi que leurs alliages présentent d'excellentes propriétés magnétiques contrairement à leurs homologues oxydes, qui peuvent être exploitées en biomédecine et pour l'enregistrement magnétique ultra-haute densité. Les nano-matériaux confinés dans les NTCs peuvent présenter des propriétés et des comportements différents par rapport aux matériaux massifs. Divers effets de confinement provenant de l'interaction entre les matériaux confinés et les cavités internes des nanotubes de carbone offrent des possibilités de réglage ou la conception de nouveaux nanocomposites. Cette thèse est consacrée à l’étude d’une nouvelle approche pour le développement de matériaux nanocomposites NPMMs@NTC et de leurs propriétés. Des NPMMs de taille et forme contrôlée de Co et de Fe ont été synthétisées avec de nouveaux ligands aromatiques comme stabilisants. Ces MMNPs ont ensuite été introduites de manière sélective dans la cavité de NTCs du fait d’interactions attractives/répulsives entre les nanotubes de carbone multi-parois fonctionnalisés et les NPMMs. Nous nous sommes ensuite intéressés à la protection de ces nanoparticules de l’oxydation par l’air. Les nanoparticules de fer confinées ont ainsi été revêtues par du polyisoprène. Pour ce faire, la surface des nanoparticules de Fe a été modifiée avec un catalyseur de polymérisation par échange de ligand, puis la polymérisation de l'isoprène a été réalisée à l'intérieur du canal des NTCs. La protection de l'oxydation par le polyisoprène a été évaluée par des mesures magnétiques après exposition à l'air. De façon tout à fait surprenante, cette étude a montré que les nanoparticules de fer les plus résistantes à l’oxydation étaient celles obtenues après échange de ligand et sans polymérisation. Dans ce cas seulement les propriétés des nanoparticules originales sont maintenues après mise à l’air. Enfin, des nanostructures (particules ou fils) magnétiques bimétalliques associant le Pt au cobalt ou au fer ont été obtenues et confinées dans les NTCs. Leurs structures chimiques ordonnées ont également été étudiées par des études de recuit thermique. Le travail développé dans cette thèse ouvre de nouvelles perspectives pour la production de nouveaux nanocomposites MMNPs@NTC résistants à l’oxydation. / Carbon nanotubes (CNTs), because of their unique properties and potential use in a variety of applications, are probably the most studied class of nanomaterials. Functionalized CNTs, which can be easily manipulated and modified by covalent or non-covalent functionalization, appear as new tools in biotechnology and biomedicine. Indeed, CNTs have optical, electronic and mechanical properties that can be exploited in biological or biomedical applications. Metallic magnetic nanoparticles (MMNPs) of the 3d series and their alloys exhibit excellent magnetic properties unlike their oxide counterparts, which can be exploited in biomedicine and ultra-high density magnetic recording. When confined in CNTs nano-materials can have different properties and behaviors compared to bulk materials. Various confinement effects resulting from the interaction between the confined materials and the internal cavities of CNTs provide opportunities for regulating or designing new nanocomposites. This thesis is devoted to the study of a new approach for the development of nanocomposite materials MMNPs@CNTs and their properties. MMNPs of controlled size and shape of Co and Fe were synthesized with novel aromatic ligands as stabilizers. These MMNPs were then selectively introduced into the cavity of CNTs due to repulsive/attractive interactions between the functionalized multi-walled CNTs and the MMNPs. We were then interested in the protection of these nanoparticles from oxidation by air. Thus, confined iron nanoparticles have been coated with polyisoprene. To do this, the surface of the Fe nanoparticles has been modified with a polymerization catalyst by ligand exchange; then, polymerization of isoprene was conducted inside the channel of CNTs. The protection from oxidation by the polyisoprene was evaluated by magnetic measurements after exposure to air. Quite surprisingly, this study showed that the iron nanoparticles the more resistant to oxidation were those obtained after ligand exchange and without polymerization. In this case only, the original properties of the nanoparticles are maintained after venting. Finally, magnetic bimetallic nanostructures (particles or rods) combining Pt and cobalt or iron were obtained and confined in CNTs. Their chemical structure orderings were also studied by thermal annealing studies. The work developed in this thesis opens up new perspectives for the production of new MMNPs@NTC nanocomposites resistant to oxidation.

Dank der hohen uniaxialen Kristallanisotropie der L10-geordneten Phase gelten nanopartikuläre FePt+C-Schichten als aussichtsreiche Kandidaten zukünftiger Datenspeichersysteme. Aus diesem Grund werden in der vorliegenden Arbeit in Kooperation mit HGST- A Western Digital Company Prototypen solcher Medien strukturell bis hin zu atomarer Auflösung charakterisiert. Anhand von lokalen Messungen der Gitterparameter der FePt-Partikel wird gezeigt, dass die Partikel dünne, zementitartige Verbindungen an ihrer Oberfläche aufweisen. Zusätzlich werden große Partikel mit kleinem Oberfläche-Volumen-Verhältnis von kontinuierlichen Kohlenstoffschichten umschlossen, was die Deposition weiteren Materials verhindert. Eine Folge davon ist die Entstehung einer zweiten Lage statistisch orientierter Partikel, die sich negativ auf das magnetische Verhalten der FePt-Schicht auswirkt. Weiterhin wird die besondere Bedeutung des eingesetzten Substrats sowie seiner Gitterfehlpassung zur L10-geordneten Einheitszelle nachgewiesen. So lässt sich das Auftreten fehlorientierter ebenso wie das L12-geordneter Kristallite im Fall großer Fehlpassung und einkristalliner Substrate unterdrücken, was andererseits jedoch zu einer stärkeren Verkippung der [001]-Achsen der individuellen FePt-Partikel führt. Abschließend wird mithilfe der Elektronenholographie nachgewiesen, dass die Magnetisierungsrichtungen der FePt-Partikel aufgrund von Anisotropieschwankungen von den [001]-Achsen abweichen können.:1 Einleitung 2 Grundlagen 2.1 Magnetische Datenspeicherung 2.1.1 Longitudinale magnetische Datenspeicherung (LMR) 2.1.2 Perpendikuläre Medien (PMR) 2.1.3 Thermisch-unterstützte Schreibprozesse (HAMR) 2.2 Das binäre Legierungssystem Eisen-Platin 2.2.1 Die L10-Ordnungsphase 2.2.2 Partikuläre FePt-Schichten 2.3 Strukturelle Besonderheiten nanoskopischer Systeme 2.3.1 Vorzugsorientierungen in polykristallinen Materialien 2.3.2 Einfluss der Substratoberfläche 2.4 Magnetische Eigenschaften 2.4.1 Magnetokristalline Anisotropie 2.4.2 Das Stoner-Wohlfarth-Modell der Eindomänenteilchen 2.4.3 Besonderheiten nanoskopischer FePt-Magnete 2.5 Prinzipien der hochauflösenden Transmissionselektronenmikroskopie 2.5.1 Bild- und Kontrastentstehung 2.5.2 Angewandte Bildgebungsverfahren 2.5.3 Elektronenholographie 3 Experimentelles 3.1 Probenherstellung mittels Kathodenzerstäubung 3.2 Präparation von TEM-Lamellen und VSM-Proben 3.3 Strukturelle Charakterisierung 3.3.1 Klassische und hochaufgelöste Transmissionselektronenmikroskopie 3.3.2 Elektronenenergieverlustspektroskopie (EELS) 3.3.3 Röntgendiffraktometrie (XRD) 3.3.4 Rasterkraftmikroskopie (AFM) 3.4 Vermessung der magnetischen Eigenschaften 3.4.1 Korrektur der Hysteresemessungen 3.4.2 Untersuchung des Wechselwirkungsverhaltens 3.4.3 Bestimmung der Anisotropiefelder 3.4.4 Ermittlung der magnetischen Phasenschiebung mittels Holographie 4 Legierungsbildung und Segregationsphänomene 4.1 Charakterisierung der Platinverteilung 4.2 Untersuchung der Kohlenstoffsegregation 4.3 Einfluss von Kupfer-Additiven 5 Untersuchung des Substrateinflusses 5.1 Einfluss der Gitterfehlpassung zu einkristallinen Substraten 5.2 Vergleich einkristalliner und polykristalliner Substrate 5.3 Einfluss von Substratkorngrenzen 5.4 Auswirkungen der Substratrauheit 6 Magnetische Charakterisierung einzelner FePt Inseln mittels Elektronenholographie 6.1 Experimentelle Bestimmung der magnetischen Phasenschiebung 6.2 Simulation der Phasenkontraste 7 Zusammenfassung / Highly textured L10-ordered FePt+C-films are foreseen to become the next generation of magnetic data storage media. Therefore prototypes of such media (provided by HGST- A Western Digital Company) are structurally investigated down to the atomic level by HR-TEM and the observed results are correlated to the magnetic performance of the film. In a first study the occurrence of a strongly disturbed surface layer with a lattice spacing that corresponds to cementite is observed. Furthermore the individual particles are surrounded by a thin carbon layer that suppresses the deposition of further material and leads, therefore, to the formation of a second layer of particles. Without a contact to the seed layer these particles are randomly oriented and degrade the magnetic performance of the media. A further study reveals, that a selection of single-crystalline substrates with appropriate lattice mismatch to the L10-ordered unit cell can be applied to avoid the formation of in-plane oriented and L12-ordered crystals. Unfortunately, the required large mismatch results in a broadening of the texture of the [001]-axes of the individual grains. As electron holography studies reveal, the orientation of the magnetization of the individual grains can differ from the structural [001]-axis due to local fluctuations of the uniaxial anisotropy.:1 Einleitung 2 Grundlagen 2.1 Magnetische Datenspeicherung 2.1.1 Longitudinale magnetische Datenspeicherung (LMR) 2.1.2 Perpendikuläre Medien (PMR) 2.1.3 Thermisch-unterstützte Schreibprozesse (HAMR) 2.2 Das binäre Legierungssystem Eisen-Platin 2.2.1 Die L10-Ordnungsphase 2.2.2 Partikuläre FePt-Schichten 2.3 Strukturelle Besonderheiten nanoskopischer Systeme 2.3.1 Vorzugsorientierungen in polykristallinen Materialien 2.3.2 Einfluss der Substratoberfläche 2.4 Magnetische Eigenschaften 2.4.1 Magnetokristalline Anisotropie 2.4.2 Das Stoner-Wohlfarth-Modell der Eindomänenteilchen 2.4.3 Besonderheiten nanoskopischer FePt-Magnete 2.5 Prinzipien der hochauflösenden Transmissionselektronenmikroskopie 2.5.1 Bild- und Kontrastentstehung 2.5.2 Angewandte Bildgebungsverfahren 2.5.3 Elektronenholographie 3 Experimentelles 3.1 Probenherstellung mittels Kathodenzerstäubung 3.2 Präparation von TEM-Lamellen und VSM-Proben 3.3 Strukturelle Charakterisierung 3.3.1 Klassische und hochaufgelöste Transmissionselektronenmikroskopie 3.3.2 Elektronenenergieverlustspektroskopie (EELS) 3.3.3 Röntgendiffraktometrie (XRD) 3.3.4 Rasterkraftmikroskopie (AFM) 3.4 Vermessung der magnetischen Eigenschaften 3.4.1 Korrektur der Hysteresemessungen 3.4.2 Untersuchung des Wechselwirkungsverhaltens 3.4.3 Bestimmung der Anisotropiefelder 3.4.4 Ermittlung der magnetischen Phasenschiebung mittels Holographie 4 Legierungsbildung und Segregationsphänomene 4.1 Charakterisierung der Platinverteilung 4.2 Untersuchung der Kohlenstoffsegregation 4.3 Einfluss von Kupfer-Additiven 5 Untersuchung des Substrateinflusses 5.1 Einfluss der Gitterfehlpassung zu einkristallinen Substraten 5.2 Vergleich einkristalliner und polykristalliner Substrate 5.3 Einfluss von Substratkorngrenzen 5.4 Auswirkungen der Substratrauheit 6 Magnetische Charakterisierung einzelner FePt Inseln mittels Elektronenholographie 6.1 Experimentelle Bestimmung der magnetischen Phasenschiebung 6.2 Simulation der Phasenkontraste 7 Zusammenfassung

The main objective of the thesis was to construct a wide-field Kerr microscope to study all-optical helicity-dependent (AOHDS) switching in FePt nanograins. The wide- field Kerr microscope was successfully implemented into AOHDS experiments, was fully characterized and optimized for maximum image contrast. The real-time imaging and resolution of 2, 5µm enables the study of a wide range of magnetic materials and their dynamics. Moreover, a new light source, the High Lumen Density MODULE from CRY- TUR, spol. s r.o., was tested for future application in Kerr microscopy. The technical solution enabled to form a collimated beam with low divergence required for Kerr mi- croscopy. From the switching experiments on FePt nanograins, we observed a strong non-magnetic contribution to the magnetic signal, not reported in previous works. The experiments have also shown that the switching intensity depends on the laser spot size and total laser power, suggesting that the FePt grains are not entirely isolated. The grains' ensemble exhibits a more complex behavior than anticipated. 1

Nous nous proposons d'étudier l'interaction au niveau de l'interface entre un matériau antiferromagnétique et un ferromagnétique par un ensemble de techniques expérimentales qui utilisent le rayonnement synchrotron. Nous sommes particulièrement intéressés par l'effet de couplage d'échange dans les couches minces magnétiques avec anisotropie hors du plan. Les systèmes que nous avons étudiés sont les couches ordonnées chimiquement, FePt et MnPt sur Pt(001), et Fe/Ag(001), éventuellement couplée à CoO. Notre approche consiste à trouver de surfaces adaptées et d'étudier, pour chaque bicouche, la croissance individuelle de chaque élément, alliage ou oxyde. A travers le contrôle d'un certain nombre des paramètres, comme la structure de la surface, la propreté, le taux et la température de déposition, nous avons obtenus une bonne connaissance du processus de croissance. Les systèmes obtenus ont été étudiés in situ par la diffraction de surfaces et ex situ par l'effet Kerr magnéto-optique, le dichroïsme circulaire magnétique de rayons X et la spectroscopie d'absorption de rayons X. La relation entre le couplage d'échange, qui se manifeste par l'augmentation de la coercivité et par un champ de décalage, et la structure des couches est discutée pour les interfaces MnPt/FePt and CoO/Fe.

Highly ordered magnetic materials with high perpendicular magnetic anisotropy (PMA), such as the L10 ordered FePt, and new recording technologies, such as bit patterned media (BPM), have been proposed as solutions to the media trilemma problem and provide promising strategies towards future high-density magnetic data storage media. L10 ordered FePt thin films can provide the necessary high PMA. However, the ordering of this material perpendicular to the plane of the films remains challenging since high-temperature and time-consuming processes are required. In this work, a remote plasma sputtering system has been used for the investigation of FePt thin films in order to understand if the greater control of process parameters offered by this system can lead to enhanced ordering in L10 FePt thin films at low temperatures compared with conventional dc magnetron approaches. More specifically, the effect of the different substrate temperatures and the target bias voltages on the ordering, the microstructure and the magnetic properties of FePt thin films was investigated. Highly ordered FePt thin films were successfully fabricated after post-annealing processes and were patterned into arrays of FePt islands. This patterning process was carried out with e-beam lithography and ion milling. Initial MFM measurements of these islands showed their single-domain structure for all the island sizes, which indicated the high PMA of the FePt. Magnetometry measurements were also carried out with a novel polar magneto-optical Kerr effect (MOKE) system which was designed and built during this project. This system has unique capabilities which are: a) the application of uniform magnetic field up to 2 Tesla, b) the rotation of the field to an arbitrary angle and c) the use of lasers of four different wavelengths. The combination of these abilities enabled measurements on ordered FePt thin films and patterned media which can pave the way for further highly sensitive measurements on magnetic thin films and nanostructures.

Scanning transmission electron microscopy (STEM) has been widely used for characterization of materials; to identify micro- and nano-structures within a sample and to analyze crystal and defect structures. High-angle annular dark field (HAADF) STEM imaging using atomic number (Z) contrast has proven capable of resolving atomic structures with better than 2 A lateral resolution. In this work, the HAADF STEM imaging mode is used in combination with multislice simulations. This combination is applied to the investigation of the temperature dependence of the intensity collected by the HAADF detector in silicon, and to convergent beam electron diffraction (CBED) to measure the degree of chemical order in intermetallic nanoparticles. The experimental and simulation results on the high–angle scattering of 300 keV electrons in crystalline silicon provide a new contribution to the understanding of the temperature dependence of the HAADF intensity. In the case of 300 keV, the average high-angle scattered intensity slightly decreases as the temperature increases from 100 K to 300 K, and this is different from the temperature dependence at 100 keV and 200 keV where HAADF intensity increases with temperature, as had been previously reported by other workers. The L10 class of hard magnetic materials has attracted continuous attention as a candidate for high-density magnetic recording media, as this phase is known to have large magnetocrystalline anisotropy, with magnetocrystalline anisotropy constant, Ku, strongly dependent on the long-range chemical order parameter, S. A new method is developed to assess the degree of chemical order in small FePt L10 nanoparticles by implementing a CBED diffraction technique. Unexpectedly, the degree of order of individual particles is highly variable and not a simple function of particle size or sample composition. The particle-to-particle variability observed is an important new aspect to the understanding of phase transformations in nanoparticle systems.

FePt films with the L10 phase have potential applications for magnetic recording and permanent magnets due to its high magnetocrystalline anisotropy energy density. Heat treatment of n multilayer films is one approach to form the L10 FePt phase through a solid state reaction. This thesis has studied the diffusion and reaction of n multilayer films to form the L10 FePt phase and has used this understanding to construct exchange spring magnets. The process-structure-property relations of n multilayer films were systematically examined. The transmission electron microscopy (TEM) study of the annealed multilayers indicates that the Pt layer grows at the expense of Fe during annealing, forming a disordered fcc FePt phase by the interdiffusion of Fe into Pt. This thickening of the fcc Pt layer can be attributed to the higher solubilities of Fe into fcc Pt, as compared to the converse. For the range of film thickness studied, a continuous L10 FePt product layer that then thickens with further annealing is not found. Instead, the initial L10 FePt grains are distributed mainly on the grain boundaries within the fcc FePt layer and at the Fe/Pt interfaces and further transformation of the sample to the ordered L10 FePt phase proceeds coupled with the growth of the initial L10 FePt grains. A comprehensive study of annealed n films is provided concerning the phase fraction, grain size, nucleation/grain density, interdiffusivity, long-range order parameter, and texture, as well as magnetic properties. A method based on hollow cone dark field TEM is introduced to measure the volume fraction, grain size, and density of ordered L10 FePt phase grains in the annealed films, and low-angle X-ray diffraction is used to measure the effective Fe-Pt interdiffusivity. The process-structure-properties relations of two groups of samples with varying substrate temperature and periodicity are reported. The results demonstrate that the processing parameters (substrate temperature, periodicity) have a strong influence on the structure (effective interdiffusivity, L10 phase volume fraction, grain size, and density) and magnetic properties. The correlation of these parameters suggests that the annealed n multilayer films have limited nuclei, and the subsequent growth of L10 phase is very important to the extent of ordered phase formed. A correlation between the grain size of fcc FePt phase, grain size of the L10 FePt phase, the L10 FePt phase fraction, and magnetic properties strongly suggests that the phase transformation of fccL10 is highly dependent on the grain size of the parent fcc FePt phase. A selective phase growth model is proposed to explain the phenomena observed. An investigation of the influence of total film thickness on the phase formation of the L10 FePt phase in n multilayer films and a comparison of this to that of FePt co-deposited alloy films is also conducted. A general trend of greater L10 phase formation in thicker films was observed in both types of films. It was further found that the thickness dependence of the structure and of the magnetic properties in n multilayer films is much stronger than that in FePt alloy films. This is related to the greater chemical energy contained in n films than FePt alloy films, which is helpful for the L10 FePt phase growth. However, the initial nucleation temperature of n multilayers and co-deposited alloy films was found to be similar. An investigation of L10 FePt-based exchange spring magnets is presented based on our understanding of the L10 formation in n multilayer films. It is known that exchange coupling is an interfacial magnetic interaction and it was experimentally shown that this interaction is limited to within several nanometers of the interface. A higher degree of order of the hard phase is shown to increase the length scale slightly. Two approaches can be used to construct the magnets. For samples with composition close to stoichiometric L10 FePt, the achievement of higher energy product is limited by the average saturation magnetization, and therefore, a lower annealing temperature is beneficial to increase the energy product, allowing a larger fraction of disordered phase. For samples with higher Fe concentration, the (BH)max is limited by the low coercivity of annealed sample, and a higher annealing temperature is beneficial to increase the energy product.

Advanced scanning magnetoresistive microscopy (SMRM) — a robust magnetic imaging and probing technique — is presented. It utilizes conventional recording heads of a hard disk drive as sensors. The spatial resolution of modern tunneling magnetoresistive sensors is nowadays comparable with more commonly used magnetic force microscopes. Important advantages of SMRM are the ability to detect pure magnetic signals directly proportional to the out-of-plane magnetic stray field, negligible sensor stray fields, and the ability to apply local bipolar magnetic field pulses up to 10 kOe with bandwidths from DC up to 1 GHz. The performance assessment of this method and corresponding best practices are discussed in the first section of this work. An application example of SMRM, the study on chemically ordered L10 FePt is presented in a second section. A constructed heater unit of SMRM opens the path to investigate temperature-dependent magnetic properties of the medium by recording and imaging at elevated temperatures. L10 FePt is one of the most promising materials to reach limits in storage density of future magnetic recording devices based on heat-assisted magnetic recording (HAMR). In order to be implemented in an actual recording scheme, the medium Curie temperature should be lowered. This will reduce the power requirements, and hence, wear and tear on a heat source — integrated plasmonic antenna. It is expected that the exchange coupling of FePt to thin Fe layers provides high saturation magnetization and elevated Curie temperature of the composite. The addition of Cu allows adjusting the magnetic properties such as perpendicular magnetic anisotropy, coercivity, saturation magnetization, and Curie temperature. This should lead to a lowering of the switching field of the hard magnetic FeCuPt layer and a reduction of thermally induced recording errors. In this regard, the influence of the Fe layer thickness on the switching behavior of the hard layer was investigated, revealing a strong reduction for Fe layer thicknesses larger than the exchange length of Fe. The recording performance of single-layer and bilayer structures was studied by SMRM roll-off curves and histogram methods at temperatures up to 180 °C In the last section of this work, SMRM advantages are demonstrated by various experiments on a two-dimensional magnetic vortex lattice. Magnetic vortex is a peculiar complex magnetization configuration which typically appears in a soft magnetic structured materials. It consists of two coupled sub-systems: the core, where magnetization vector points perpendicular to the structure plane, and the curling magnetization where magnetic flux is rotating in-plane. The unique properties of a magnetic vortex making it an object of a great research and technological interest for spintronic applications in sensorics or data storage. Manipulation of the vortex core as well as the rotation sense by applying a local field pulse is shown. A spatially resolved switching map reveals a significant "write window" where vortex cores can be addressed correctly. Moreover, the external in-plane magnet extension unit allow analyzing the magnetic vortex rotational sense which is extremely practical for magnetic coupling investigations of magnetic coupling phenomena.

This work has been devoted to the study of phase transformations involving chemical ordering and magnetic properties evolution in bulk Fe-Pt alloys composed of nanometer-sized grains. A comprehensive study of phase transformations and ordering in Fe-Pt alloys is performed by a combination of in-situ neutron powder diffraction and thermal analysis. The dependence of ordering processes on the alloy composition and initial microstructure (homogeneous A1 phase or multilayer-type) is established. Through the use of mechanical alloying and subsequent heat treatment it has been possible to achieve the formation of chemically highly ordered L10 FePt and, in the case of the Fe-rich and Pt-rich compositions, L12 Fe3Pt and FePt3 phases, respectively. Whereas in Pt-rich alloys the decoupling effect of the FePt3 phase leads to coercivity improvement, in Fe-rich nanocomposites a peculiar nanometer scale multilayer structure gives rise to remanence enhancement due to large effects of exchange interactions between the crystallites of the phases. The structure, magnetic properties and magnetisation reversal processes of these alloys are investigated. Experimentally observed phenomena are understood on the basis of a simple two-particle interaction model. Neutron diffraction has also been used for the investigation of the magnetic structure of ordered and partially ordered nanocrystalline Fe-Pt alloys. It has been shown that the magnetic moment of Fe atoms in L10-type Fe Pt alloys is sensitive to the compositional order. The results are compared to density functional calculations.